A coke oven is used for making coke from coal. The crushed coal is indirectly heated to generally above 1500.degree. F. in an oxygen deficient, pressurized atmosphere by heating the sidewalls of the oven with oil, gas or coal. Coal tar volatiles are driven-off from the coal, leaving behind coke for use in making iron and steel. The liberated coal tar volatiles are separately collected and distilled to provide as a by-product a basic raw material for the chemical industry.
One of the problems with coke ovens has been loss of coal as carryover fines. That is, fine coal particles, in various stages of devolatilization and carbonization, are emitted from the coke oven through the exit mains with the volatiles. In a conventional oven, the amount of carryover fines usually runs between 50 and 100 pounds per charge. These carryover fines are generally collected in a dust collector, wet scrubber and pitch trap along with some coal tar and ash, and disposed of as waste.
Recently, it has been found that the efficiency of coke ovens can be substantially improved by preheating the coal. Preheating also permits utilization of lower grades of coal and provides an improved coke product. Preheating involves flash heating typically pulverized coal to about 500.degree. F. before charging to the coke oven. The coking can thus be done faster with added production for a given size coke oven, while producing a stronger more consistent coke composition from coal blends comprising large portions of low grade coal. Preheating is credited with providing greater assurance of complete coking and also reducing air polluting emissions that take place when green coke is pushed.
The problem is that preheating also causes even larger losses of coal due to increases in carryover fines. The grinding of coal to fine particles and the explosive force exerted on flash heating produce this increased carryover. The high and efficient yield of usable coke from low-grade coal, however, more than offsets the losses in carryovers fines. It has been suggested to use clarifiers to retrieve coal fines trapped by spray water and recycle these to the preheater, but this procedure adds to the cost of installation and operation and still results in loss of substantial carryover fines.
A compounding problem with coke ovens is air pollution during charging. It has been estimated that 70 percent of all emissions from a coke oven occur during charging. A prominent solution for this problem, which has been commercially used, is pipeline charging. Pipeline charging involves grinding and screening the coal to form a mass of coal particles with a maximum size of 1/4 inch and most under 1/8 inch. This mass can be pressurized typically by steam and propelled through pipes to charge the coke oven. The big attraction for this system is that the coal enters the ovens through a closed stationary network, with little opportunity for dust emissions.
Pipeline charging, however, further increases the tendency of fine coal particles to carryover to the oven exhaust system. With preheating, anywhere between 200 and 1000 pounds of carryover fines will typically flow into the exit mains during a pipeline charge, with the equipment designed to handle 2000 pounds of carryover fines. See Iron Age (Mar. 1, 1976) pp. MP-9 to 12. The stringencies of recent environmental controls on coke ovens require these losses be accepted.
The present invention overcomes these difficulties and disadvantages. It provides a closed coke oven system where substantially all carryover fines are reclaimed and recycled to the coke oven with minimum cost in equipment and operation. The most pertinent art is believed to be the disclosures of U.S. Pat. Nos. 3,268,071, 3,617,228, 3,637,464, and 3,665,066 and Canadian J. of Chem. Eng., Vol. 54 February/April 1976, pp. 3-12, which have little or no relation to the present coke oven system.